From https://www.westonaprice.org/health-topics/food-features/broth-is-beautiful/#gsc.tab=0

Photo by Nathan Dumlao on unsplash.com 

Attention to Detail

Stock or broth begins with bones, some pieces of meat and fat, vegetables and good water. For beef and lamb broth, the meat is browned in a hot oven to form compounds that give flavor and color–the result of a fusion of amino acids with sugars, called the Maillard reaction. Then all goes in the pot–meat, bones, vegetables, and water. The water should be cold because slow heating helps bring out flavors. 

Heat the broth slowly and once the boil begins, reduce heat to its lowest point, so the broth just barely simmers. Scum will rise to the surface. This is a different kind of colloid, one in which larger molecules–impurities, alkaloids, large proteins called lectins–are distributed through a liquid. One of the basic principles of culinary art is that this effluvium should be carefully removed with a spoon. Otherwise, the broth will be ruined by strange flavors. Besides, the stuff looks terrible. “Always Skim” is the first commandment of good cooks. 

Two hours simmering is enough to extract flavors and gelatin from fish broth. Larger animals take longer–all day for broth made from chicken, turkey, or duck and overnight for beef broth. 

Broth should then be strained. The leavings, picked over, can be used for terrines or tacos or casseroles. Perfectionists will want to chill the broth to remove the fat. Stock will keep for several days in the refrigerator or may be frozen in plastic containers. Boiled down it concentrates and becomes a jellylike fumée or demi-glaze that can be reconstituted into a sauce by adding water.